1. Field of the Invention
The present invention relates to a polyamide resin composition comprising an aromatic polyamide resin, a modified polyphenylene ether resin, a liquid-crystalline polymer, a compatibilizing agent and, as an inorganic filler, a carbon fiber, mica, glass flake or the like. More particularly, the present invention is concerned with a polyamide resin composition excellent in water resistance, chemical resistance, mechanical properties, fluidity during molding and rigidity and less liable to cause warpage, and a light-weight housing for electronic equipment, such as personal computers and word processors, produced by molding the polyamide resin composition.
2. Description of Related Art
At the present time, there is all ever-increasing demand for a reduction in the size and a reduction in the weight of electronic equipment, such as domestic electric appliances, telephones and OA equipment, and the use of plastics for housing of this equipment has been studied in the art.
In particular, note book size personal computers and word processors among the above-described types of portable OA equipment are required to have a reduced size and a reduced weight due to an increase in the range of use thereof.
This has caused the material for use in the portable equipment to become changed from metals to plastics.
The plastics currently used in the art, however, cannot attain a satisfactory reduction in the weight, so that a further reduction in the weight is desired in the art.
ABS resins, AS resins, polycarbonate resins, modified polyphenylene ether resins, etc., have hitherto been studied as plastics for a housing from the viewpoint of a good moldability, a low cost and a good appearance in the form of finished articles. However, they are unsatisfactory in rigidity, so that no housing suitable for practical use has been provided in the art.
Further, in the housing for portable electronic equipment, the plate thickness should be as small as possible, and housing having a plate thickness of 3 to 5 mm and about 2 mm for a thin plate has been put to practical use.
The reduction in the plate thickness for the plastic housing to 2 mm or less unfavorably leads to the following problems.
(a) Rigidity
Since the conventional resins, such as ABS resin, has a low rigidity, if they, as such, are molded into molded articles having a small plate thickness., the molded articles are largely deformed upon being exposed to a small external force, so that they cannot protect contents of the housing, such as liquid crystal display panels comprising glass, printed circuit boards and keyboards.
(b) Fluidity during Molding
When a housing having a small thickness and a large area is molded, the resin is rapidly cooled in a mold, which increases the viscosity of the resin. This often causes the flow of the resin to stop, so that short molding occurs.
(c) Warpage
Even though short molding does not occur, the incorporation of a fibrous filler for the purpose of increasing the strength in addition to molding under high pressure gives rise to warpage due to the anisotropy of mold shrinkage derived from the orientation of the fiber.
Impact Strength
In molded articles having a small plate thickness, an increase in the rigidity of the material for the purpose of preventing the deformation generally lowers the impact strength. In portable electronic equipment, such as note book size personal computers and word processors, there are occasions where accidental dropping occurs. The dropping often gives rise to crazing, cracking, etc. of the housing.
since the specific gravity of the plastics is generally in the range of from 0.9 to 2.0, a further reduction in the weight through a reduction in the specific gravity cannot be expected. Therefore, a reduction in the plate thickness of the housing is necessary for a further reduction in the weight.
However, the molding of the conventional resin composition into a housing having a plate thickness of 2.0 mm or less leads to the above-described problems.
Japanese Unexamined Patent Publication (Kokai) No. 4-154864 discloses a polyamide resin composition comprising a polyamide resin, a modified polyphenylene ether resin, a carbon fiber and mica that is described as a material for a housing having a small plate thickness for portable equipment capable of overcoming the above-described problems of the prior art. However, also in this conventional composition, when molding of a molded article having a very small thickness of about 1 mm is intended, it is necessary to set the molding (resin) temperature at a high value close to the decomposition temperature of the resin, to provide many gates or to increase the runner diameter for the purpose of providing fluidity. This unfavorably gives rise to the occurrence of deterioration in the resin and defects of gas accumulation in the molded article. This has an adverse effect on the molded article and increases defective moldings and, hence, leads to an increase in the cost. Moreover, in the molding of a molded article having a very small thickness, since the injection pressure is higher than usual molding, particularly, warpage is unfavorably liable to occur.
On the other hand, liquid-crystalline polymers (thermotropic liquid-crystalline polymers) have a rigid molecular structure, form a liquid crystal in a molten state, are easily orientated by shear force applied during molding, exhibit a good fluidity during molding and can provide molded articles having a high strength, a high flexural strength, a high modulus of elasticity, an excellent impact resistance and a small coefficient of linear expansion.
An attempt has been made to modify non-liquid-crystalline polymers by taking advantage of the above-described properties of the liquid-crystalline polymers by blending the liquid-crystalline polymer with the non-liquid-crystalline polymer. For example, in Polymer Engineering & Science, 27, p. 410 (1987), C. Kiss et. al. reports that blending of a liquid-crystalline polymer with conventional engineering plastics followed by molding causes the liquid-crystalline polymer to be dispersed in a fibril form in the non-liquid-crystalline polymer and the modulus of elasticity can be improved by virtue of the reinforcing effect of the liquid-crystalline polymer. In Plastic Engineering, p. 39 (1987) , T. S. Chung reports that blending of a liquid-crystalline polymer with nylon 12 followed by molding provides a molded article having a small coefficient of linear expansion. Further, Japanese Unexamined Patent Publication (Kokai) No. 56-115357 discloses that the fluidity can be improved by blending a liquid-crystalline polymer with a non-liquid-crystalline polymer.
However, no attempt has been made to provide a molded article less susceptible to warpage often caused in molding of an article having a small thickness by enabling the molding resin temperature and the filling pressure during molding to be lowered by blending a liquid-crystalline polymer and a compatibilizing agent with a resin composition a polyamide resin containing a m-xylylene group, a modified polyphenylene ether resin, a carbon fiber, mica, a glass flake, etc., and further to apply the resultant molded article as a housing for electronic equipment, such as personal computers and word processors.
An object of the present invention is to overcome the above-described drawbacks of the prior art and to provide a molding resin composition for use as a housing material for a housing having a small plate thickness for portable equipment that is excellent in water resistance, chemical resistance, fluidity during molding, mechanical properties and rigidity and less liable to cause warpage, and a housing having an average plate thickness of 2 mm or less produced by molding this polyamide resin composition and for use in electronic equipment, such as personal computers and word processors.